Photoelectric tube



April 20, 1937. A..J. MCMASTER PHOTOELECTRIC TUBE Filed June 14, 1950 Patented Apr. 2 0, 1.9374` .UNITED STATES PATENT OFFICE 2,077,634 Pno'roELEc'rnrc TUBE Archie-J. McMaster, Chicago, Ill., assigner to G-M Laboratories, Inc., Chicago, Ill., a corporation of Illinois` Application'Jne 14, 1930, Serial No. 461,120

' 7 Claims.

'10 power output of the photoelectric tube desirable.

Furthermore, unless the energy delivered by a photoelectrlc tube is above the noise level, that is, unless the energyis greater than the microphonic currents set up byl vibration of theap- "1'5 paratus, such currents will `be amplied to the same extent as the signal current, and thus introduce serious distortion in the signal.

It has been found that the energy emitted by the cathode of a pho-isoelectric tubein response to light follows the following mathematical ex 20 pression, hn=1/2 me? plusl `w in which h is-Planks constant, n is .the frequency of light waves, and hn is energy of 25 one quantum of light. vOn the other sideof the equation m is the mass of an electron, o its velocity, and w the work or energy required to separate an electron from-its atom or the work function of `the emitting surface. If, therefore, othe work function is reduced more energy will be available for conversion to kinetic energy of lthe electronwhich means greater emissivit'y of the cathode surface.

In accordance with the general features of the 35 invention I provide means for reducing the work i 45 A further object is to provide a tube in which the retarding effect of the space-charge is eliminated.

Other objects and advantages will appear as r the description proceeds.

Referring to the drawing:

Fig. 1 is a side elevation of a tube embodyingv the invention,

Fig. 2 is a view similar to Fig. 1 showing the 55; cell rotated.- through an .angle'of 90 degrees,

Fig. 3 is an enlargedsectional fragmentary view taken on line 3-3 of Fig. 1, and,

Fig. 4 is a view similar to Fig. 3, illustrating a modified form of the invention.

In practicing the invention a base 6 is provided 5 having an envelope 'Il mounted thereon, and within the envelope is a stem or press 8, havling a pair of. supports 9 and II mountedthereon. Supports 9 and H carry a semi-cylindrical y plate I3 which 'is processed to provide a light '-10 vsensitive surface.` It has been found that the -electron emitting surface of a photoelectric tube should be extremely thin. However, a -thin 'photoelectric surface has a low conductivity and 'it is difficult to make good electrical contactvm therewith, and therefore, I provide a conducting base such as copper, silver, magnesium, gold, or

other low resistance metals underneath the photoelectric surface. While the plate I3 may be made of the materials enumerated above, it is preferably made by silver plating a copper plate.

This plate has a bead 24 lof an insulating ma.-

terial such as an insulating cement secured thereto, and anchored in the end of the bead is a conductor 25 which extends through the stem 25 of the tube, for permitting electrical contact to be made therewith. Forwardly of the plate is a filament 26 which is coated with silver or other low boiling point metal which may be vaporized.

Secured to one conductor of the filament is aJ '3u disc or screen 21 for preventing the vaporizing silver from being deposited upon the inner wall of the envelope, as will be described later.

Simultaneously with the mounting of the elements in theenvelope, a capsule or pellet |11 is. 1.

mounted in the upper portion of the envelope on a support I8, whichis secured to the upper end.

of the'anode by welding or the like. The pellet consists of a disc I9 having a bead 2l of an 'alkali or-alkaline earth metal salt mixed with' calcium or some other element having a higher afnity for the radical or'element of the salt than the alkali or alkaline earth metal, from which the alkali or alkaline earth metal is evapo rated and deposited upon the cathode. If, for. instance, we wish to make a caesium tube, the pellet may consist of a mixture of caesium chloride and calcium. This mixture, upon heating the pellet, forms calcium chloride and llberates caesium. The particular alkali or alkaline earth metal salt used' depends upon the characteristics desired in the tube. If we wish to obtain a tube which is highly sensitive to light in the red and yellow regicnsof the spectrum we use a caesium 'salt such as caesium chloride, caesium carbonate,

caesium nitrate, caesium trinitride, caesium silicate, or caesium azide. If we wish to obtain a tube responsive to shorter wave lengths we use potassium chloride, potassium carbonate, potassium nitrate, potassium trinitride, potassium silicate, or potassium azide. For still shorter wavelengths salts of the alkaline earth metals are used such as barium, strontium, and magnesium carbonates. All of these metals are high in the electromotive series.- The method of processing the silver plated copper plate in the tube will first be described in connection with the caesium salts. After the elements and the pellet of caesium salt have been mounted in the envelope, the envelope is placed on an evacuating pump and exhausted. At the same time the envelope or bulb is placed in an oven to heat the bulb and expel the gases therefrom, and also to preliminarily expel the gases from the elements within the bulb. The cathode is then inductively heated by means of a high frequency current coil which is associated therewith in such a manner that the major portion of the lines of ux link with the portion of the plate adjacent the stem in order to avoid overheating of the disc I9 supporting the pellet. After the bulb and elements have been thoroughly heated and exhausted to remove the gases from the tube, the envelope and elements are allowed to cool while the evacuation is maintained.'

Whenv the elements and bulb have been sumciently cooled, about one totwo millimeters of oxygen is admitted into the bulb and a glow dis-- o and converts the silver on the cathode to a silver oxide. n account of the roughe'ned surface of the silver plate obtained as pointed out above, the oxidation is enhanced and covers a larger area than would be'the case upon a perfectly smooth surface. 'I'he cathode being thoroughly oxidized the bulb is again evacuated and metallic caesium is evaporated from the pellet by inductively heating disc I9. It will be noted that the disc is at right angles to the axis of the cathode, and therefore, the pellet may be heated without causing much heating of the cathode, and the cathode being cooler than the disc, the metallic caesium tends to deposit upon the cathode forming a layer I5. In order to prevent the caesium u from being deposited on the inner walls of the bulb, the bulb is heated during this operation in an oven to a temperature of from 150 to 170 C'. During this step evacuation is maintained and the bulb remains considerably hotter than the cathode. 'I'he heating of the bulb is continued until no excess caesium is left in the tube. However,

care must be taken not to applytoohighatemperature to the bulb as it would tend to again liberate the caesium which has been deposited on the 65 cathode. In this step the caesium is deposited in a thin llm on the silver oxide and some of the caesium combines with oxygen liberated by the silver oxide to form caesium oxide. It was stated that the heating and evacuation are continued 70 until no excess caesium isv left in the bulb. Some of the caesium appears to be absorbed by the caesiumoxide and the Abest results are obtained when ,the layer of metallic caesium thereon is as thin as possible. It appears that the light sensitive characteristics of the cathode are probably due to metallic caesium, and the caesium absorbed by the underlying caesium oxide seems to replace the caesium which leaves the cathode. The metallic caesium appears to be present as a. thin adsorbed film on the surface of the cathode and this film serves as the light sensitive electron emissive agent.

The above step completes the cathode, and the anode is formed by depositing a thin translucent film I6 of silver upon the cathode. This is done by passing an electrical current through the filament 26, causing the silver coating thereof to be vaporized and the vaporized metallic silver collects upon the plate cathode as well as the insulating bead 24, thereby conductively connecting the anode with conductor 25. By thus placing the anode in direct contact with the electron emissive surface, the energy required to Withdraw the electrons from the cathode, or in other words, the work function of the cathode surface, is greatly reduced. When the cathode and anode are separated by a space, the electrons in the intervening space form a space charge which tends to repel the electrons and prevent them from leaving the cathode. It will be seen that by means of the present invention the space charge is completely eliminated. 'I'his tube may be used either with or without an external source of energy.

Fig. 4 represents a slightly modified form of the invention, in which the insulating bead 2l' has a layer of conducting material 28, such as a conducting cement covering the upper surface thereof; This covering does not extend to the cathode but is insulated therefrom by the vbase of the bead. 'I'he conductor 25' is anchored in the insulating bead and is conductively connected to the covering 28. In this manner the contact area between the thin film of the anode and the con-,l ductor is made much larger. The lament 26' in this modification is surrounded by a thimble 21', which is open toward the cathode and tends to direct the vaporized silver more effectively upon the cathode and particularly the insulating bead to insure a good electrical connection between the anode and conductor 25'.

It will be understood that the silver anode layer I6 need not be translucent in the sense that it diifuses or disperses the light which it transmits. It is sufiicient that the layer be permeable to the radiations which are to be used for actuating the device. Obviously the device is not limited to use with visible radiations. In the claims I have employed the term light permeable as meaning permeable to the radiations which are to actuate the device whether they include visible radiations .or not.

It will be understood that the nature and embodiments of the invention herein described and disclosed are merely illustrative and that many changes and modications may be made therein without departing from the spirit and scope of the invention.

What I claim is new and desire to protect by Letters Patent of the United States is:

1. A photoelectric tube comprising a silver surface cathode, an insulating bead` secured thereto, a conductor anchored in said bead and insulated from said cathode, an oxide coating on said cathode surface, said coating including an oxide of an alkali metal, an alkali metal deposit on said coating and a thin silver deposit coating said alkali metal and said bead and making contact` with said conductor.

2. A photoelectric tube comprising an envelope, a cathode comprising a metallic plate, a metallic oxide coating on said plate, said metallic oxide including an alkali metal oxide and a metallic alkali metal deposit on said oxide, a conductor anchored to said plate but insulated therefrom, a light-permeable metallic anode electrically connected to said conductor, said anode being in contact with said alkali metal deposit.

3. A photoelectric tube comprising an envelope, a silver surface plate cathode mounted therein, a silver oxide coating on said surface, an alkali metal deposit on said oxide, an insulating bead mounted on the cathode, a conductor anchored thereto, a conducting material on the upper portion of said bead andjy contacting said conductor, and an anode layer overlying said alkali metal and contacting said conducting material, said layer being substantially imperforate and light permeable.

4. A photoelectric tube comprising an envelope,

20 a metallic plate cathode mounted therein, a light sensitive surface including an alkali metal thereon, an insulating bead on said cathode, a light permeable anode in contactwith said light sensitive surface and a conductor insulated from 25 said metallic plate cathode and anchored in said bead and electrically connected to said anode.

5. A photoelectric tube comprising an envelope, a metallic plate cathode mounted therein, a light sensitive surface including an alkali metal thereon, an insulating bead on said cathode, a conducting material on the upper portion of said bead, a light permeable anode in contact with said light sensitive surface and electrically connected to said material, and a conductor connecting to said material.

6. A photoelectric tube comprising an envelope, a metallic plate cathode mounted therein, a light-sensitive material thereon, an insulating bead on said cathode, a light-permeable anode in contact with said light-sensitive material, and a conductor insulated from said metallic plate cathode and anchored in said bead and electrically connected to said anode.

7. A photoelectric tube comprising an envelope, a metallic plate cathode mounted therein, a lightsensitive material thereon, an insulating bead on said cathode, a conducting material on the upper portion of said bead, a light-permeable anode in contact with said light-sensitive material,

and electrically connected to said material, and ar conductor connected to said material.

ARCHIEl J. MCMASTER. 

